1. Field of the Invention
This invention relates to processes for preparing saponified ethylene-vinyl acetate copolymer, and more particularly, to processes which simultaneously effect the saponification and dispersion of ethylene-vinyl acetate copolymer to fine particulate form.
2. Description of the Prior Art
Processes for simultaneously dispersing and saponifying ethylene-vinyl acetate (EVA) copolymers to provide particulate hydrolyzed ethylene-vinyl acetate (HEVA) copolymers are known. In German Democratic Republic (DDR) Patent No. 88,404, there is described a process for simultaneously dispersing and saponifying EVA copolymers employing sodium hydroxide or potassium hydroxide as the saponification agent and an alkyl sulfonate, an acyl derivative of N-methyltaurine, a higher fatty acid soap, an alkaryl sulfonate or a nonionic surface-active agent derived from ethylene oxide as the dispersion agent.
The process described involves saponifying ethylene-vinyl acetate copolymers at elevated temperature and pressure including, as the final step, discharging the reaction mixture at the operating temperature and pressure directly into a quench vessel at atmospheric or subatmospheric pressure. The quench vessel contains water that is stirred during the discharge operation and the rate of discharge of the reaction mixture is regulated by means of a needle valve. Thus, the sudden release of the reaction mixture causing a portion of the reaction medium to vaporize apparently results in formation of the dispersion due to the atomizing effect of the needle valve. This patent also discloses the optional use of dispersants, but it is apparent from the data provided that such dispersants have only a secondary effect, the primary determinant of dispersion being the discharge of the hot reaction mixture to the quenching bath. From the particle size distribution data provided in the disclosure, it is clear that the presence of dispersing agent seems to favor smaller particles, but is not absolutely essential since comparable dispersions are obtained when dispersing agents are not present in the reaction mixture. There is no indication that a dispersion of the polymer occurs in the reaction mixture prior to discharge when dispersing agents are present but the data provided shows that, on discharge, a dispersion is produced in the presence or absence of dispersing agent. Attempts to obtain dispersions of saponified EVA using N-oleoylsarcosinate as dispersing agent by merely cooling the reaction mixture without the described discharge step of DDR No. 88,404 have not produced dispersions. Similarly, when arylsulfonate dispersants are employed in lieu of the sarcosinate, no dispersions are obtained when the reaction mixture is cooled. Thus, it must be concluded that dispersion only occurs on discharge.
The dispersed product obtained by the method of No. 88,404 is of fairly large particle size, the heavy majority of the particles being of diameters greater than 0.125 mm, i.e., usually over 80% of the dispersed particles. In addition, the product is composed of irregular particles, with no spherical particles being observed.
It has also been determined that certain other known and conventional dispersants are not effective in the production of such dispersions of EVA copolymers. The condensed aryl sulfonate salt dispersants are not effective in the dispersion to fine, particulate form of saponified ethylene-vinyl acetate polymers from saponification reaction mixtures. Experience to the present time with these dispersing agents has shown little, if any, tendency to produce a dispersion of the saponified product, the product obtained being a continuous mass which adhered to the walls of the reactor and the agitator. Similarly, the ethylene oxide-propylene oxide block copolymer dispersing agents described in U.S. Pat. No. 3,422,049 have been found to be ineffective in producing fine dispersions of saponified ethylene-vinyl acetate copolymers except where the residual salt content of the saponified product is reduced to not more than 0.1% calculated as cation as described in copending U.S. Pat. No. 4,027,079.
In contrast, the present invention provides a product at rapid rates with excellent reproducibility whose particle size can be controlled by regulating the amount of soap dispersant employed. Moreover, when a water-soluble, substantially neutral salt is also present during the dispersion-saponification process, it in itself favors the formation of finer particles and promotes particle sphericity.
Although the present invention is not restricted to the production of spherical particles, and for some purposes spherical particles are not necessary or advantageous, it is nevertheless true that spherical particle shape is often of substantial importance in the commercial applications of the present products, contributing superior fluidization characteristics, shorter melting time, and improved dispersibility to pulverulent compositions in which they are used. Thus, the spherical shaped particles are superior in powder form for static or fluidized dip coating, spraying, dusting and flame spraying applications as well as for preparing relatively stable dispersions in water or other liquids for use in roller, dip or spray coating.